1. Field of the Invention
The present invention relates to an ionically conductive material, to its preparation and to its uses.
2. Description of the Background
Electrochemical systems for energy storage, for example batteries or supercapacities which operate with high elementary voltages, demand electrolytes which have an extensive range of stability. Such electrolytes are obtained by dissolving one or more solutes (1/mM').sup.+ X'.sup.- in a dipolar liquid solvent, a solvating polymer or their mixtures. M' is a cation of valency m, such as a proton, a cation derived from a metal (for example Li, Na, K, Mg, Ca, Cu, Zn or La) or an organic cation such as an ammonium ion, a guanidinium ion, a phosphonium ion or a sulphonium ion. Ionic compounds (1/mM').sup.+ X'.sup.- in which the anion X'.sup.- has a delocalized charge exhibit a high ionic conductivity. Among the X'.sup.- anions with a delocalized charge there may be mentioned I.sup.-, ClO.sub.4.sup.-, AsF.sub.6.sup.-, PF.sub.6.sup.-, R.sub.F SO.sub.3.sup.-, (R.sub.F SO.sub.2).sub.2 N.sup.-, R.sup.1 CO(CF.sub.3 SO.sub.2).sub.2 C.sup.- and R.sup.1 SO.sub.2 (CF.sub.3 SO.sub.2).sub.2 C.sup.-, R.sub.F denoting a perfluoroalkyl radical or a perfluoroaryl radical, R.sup.1 denoting a hydrogen atom, an alkyl radical, an oxaalkyl radical, an azaalkyl radical, an aryl radical, a perfluoroalkyl radical or a perfluoroaryl radical.
Although the abovementioned compounds may have high ionic conductivities, their use presents disadvantages. Iodide ions are easily oxidizable. Arsenic derivatives are toxic and perchlorates are explosive. Anions such as PF.sub.6.sup.-, capable of easily releasing a Lewis acid (PF.sub.6.sup.- .fwdarw.PF.sub.5) compromise the stability of the corresponding electrolytes via reactions causing the formation of electrophilic species of carbocation type.
Anions containing R.sub.F SO.sub.2 -- perfluorosulphonyl groups, in particular perfluorosulphonates R.sub.F SO.sub.3.sup.- and perfluorosulphonylimides (R.sub.F SO.sub.2).sub.2 N.sup.-, are stable and have low toxicity and the use of the corresponding ionic compounds has become generalized, especially for electrochemical generators including negative electrodes consisting of metallic lithium, a lithium alloy or a carbon-lithium intercalation compound. The preparation of these ionic compounds is, however, very costly, and the manufacture of compounds containing at least two perfluorosulphonyl groups very particularly so. Furthermore, these compounds have a high molecular mass and the mass fraction for a given molality in a solvent is large.
Compounds corresponding to the formula (1/mM).sup.+ FSO.sub.2 NSO.sub.2 F!.sup.- are known where (1/mM).sup.+ =H.sup.+, K.sup.+, Cs.sup.+, Rb.sup.+, Ag.sup.+ and (CH.sub.3).sub.4 N.sup.+ J. K. Ruff, Inorg. Chem. 4, 1446, (1965)!. In general, a salt constitutes an electrolyte which is proportionally better the lower the basicity of its anion. The basicity of the FSO.sub.2 NSO.sub.2 F!.sup.- anion is a priori higher than that of an R.sub.F SO.sub.2 NSO.sub.2 R.sub.F !.sup.- anion in which R.sub.F is a perfluoroalkyl group, because of the retrocession of the free pairs of the fluorine on the sulphur atom. Many publications report the fact that the substitution of a perfluoroalkyl group for a fluorine atom in an organic compound which is acidic in nature decreases the strength of the acid. (G. Paprott & K. Seppelt, J. Am. Chem. Soc., 106, 4060 (1984); E. D. Laganis, D. M. Lema, J. Am. Chem. Soc., 102, 6634 (1984); F. J. Bordwell, J. C. Branca et al., J. Org. Chem., 53, 780, (1988)).
Furthermore, it is known that the fluorine atom of an F--S bond is particularly labile and especially hydrolysable in the presence of water or of nucleophilic bases. Because of these disadvantages the use of the acid FSO.sub.2 NSO.sub.2 F!H as protonic electrolyte in fuel cells is not recommended M. Razak et al., J. Appl. Electrochem., 17 (5), 1057 (1987)!. On the other hand, the stability of the compounds R.sub.F SO.sub.2 NSO.sub.2 R.sub.F !H such as H(CF.sub.3 SO.sub.2).sub.2 N or H(CF.sub.3 SO.sub.2 NSO.sub.2 C.sub.4 F.sub.9) has been demonstrated M. Razak et al., op. cit.; M. Razak et al., J. Appl. Electrochem., 136, 385 (1989)!.
It is also known that the compounds H(FSO.sub.2).sub.3 C are hydrolysed spontaneously, whereas their homologues (1/mM').sup.+ (R.sub.F SO.sub.2).sub.3 C!.sup.- have been proposed as electrolyte solutes for electrochemical generators. However, just as in the case of the imides, the molecular mass and the costs of manufacture of the compounds 1/mM).sup.+ (R.sub.F SO.sub.2).sub.3 C!.sup.- are high and render their use of little interest.
JP-A-05 283 086 relates to a battery in which the electrolyte contains a cyclic ether as solvent and a salt including at least two R.sub.F SO.sub.2 groups, R.sub.F being a fluorine atom or a fluoroalkyl group. The use of salts containing two fluoroalkyl groups is described and illustrated by concrete examples relating to lithium bis(trifluoromethanesulphonyl)methanide. It is explained that a salt of the (CF.sub.3 SO.sub.2).sub.2 NLi or (CF.sub.3 SO.sub.2).sub.3 CLi type gives a conductivity which is higher when compared with a CF.sub.3 SO.sub.3 Li salt owing to the fact that the presence of a single electron-withdrawing group on the atom adjoining the lithium atom in CF.sub.3 SO.sub.3 Li increases the electron density on this atom, oxygen in this case, and therefore renders ionization, that is to say the release of Li.sup.+, more difficult, whereas in a compound (CF.sub.3 SO.sub.2).sub.2 NLi or (CF.sub.3 SO.sub.2).sub.3 CLi the presence of two electron-withdrawing groups on the atom adjoining the lithium decreases the electron density on this atom and therefore promotes the release of the Li.sup.+ ion. No information is given on salts including one or two FSO.sub.2 groups. Furthermore, the conclusions drawn from the comparison between a compound R.sub.F SO.sub.3 Li and a compound (R.sub.F SO.sub.2).sub.2 NLi or (R.sub.F SO.sub.2).sub.3 CLi when R.sub.F is CF.sub.3 cannot be simply extrapolated to the corresponding compounds in which R.sub.F is F. In fact, a compound FSO.sub.3 Li is not stable in solution in a cyclic ether, in which it decomposes to give LiF and SO.sub.3, thus causing a polymerization of the ether, in particular in the case of cyclic acetals. This compound was therefore absolutely not usable as salt in an ether. Consequently, it was not obvious that the improvement in the conductivity by the replacement of CF.sub.3 SO.sub.3 Li (which is a usable salt, even though not the most effective one) by (CF.sub.3 SO.sub.2).sub.2 NLi or (CF.sub.3 SO.sub.2).sub.3 CLi could be transposed to the case of the replacement of FSO.sub.3 Li (which is an unusable salt) by (FSO.sub.2).sub.2 NLi or (FSO.sub.2).sub.3 CLi.